I Kay Oh

Calculus Level 5

0 1 x 3 ln 2 ( x ) ( x 4 + 1 ) 2 d x = A π B C \large{\displaystyle \int^{1}_{0} \dfrac{x^3 \ln^2 (x)}{(x^4+1)^2} \, dx=\dfrac{A \pi^{B}}{C}}

where A , B , C A,B,C are positive integers and A , C A,C being coprime. Find A + B + C A+B+C .


The answer is 387.

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2 solutions

Harsh Shrivastava
Jan 21, 2016

I = 0 1 x 3 ln 2 ( x ) ( x 4 + 1 ) 2 d x \large{\displaystyle I = \int^{1}_{0} \dfrac{x^3 \ln^2 (x)}{(x^4+1)^2} }dx

Substitute x 4 = t x^{4} = t , 64 I = 0 1 ln 2 ( t ) ( t + 1 ) 2 d t \displaystyle 64I = \int^{1}_{0} \dfrac{\ln^2 (t)}{(t+1)^2}dt

Now using 1 ( 1 + t ) 2 = 1 2 t + 3 t 2 4 t 3 + \dfrac{1}{(1+t)^{2}} = 1-2t+3t^{2}-4t^{3}+ \cdots (If you want proof for this,ask me in the comments section.) \small \color{#D61F06}{\text{(If you want proof for this,ask me in the comments section.)}}

64 I = r = 0 ( 1 ) r ( 1 + r ) 0 1 t r ln 2 ( t ) d t \displaystyle 64I = \sum_{r=0}^{\infty} (-1)^{r} (1+r) \int_{0}^{1} t^{r} \ln^2(t) dt

Now using the substitution t = e x r , 0 1 t r ln 2 ( t ) d t = 2 ( 1 + r ) 3 \text{using the substitution } t = e^{\dfrac{-x}{r}},\displaystyle \int_{0}^{1} t^{r} \ln^2 (t) dt = \dfrac{2}{(1+r)^{3}}

Therefore I I becomes 32 I = r = 0 ( 1 ) r ( r + 1 ) 2 32I = \sum_{r=0}^{\infty} \dfrac{(-1)^{r}}{(r+1)^{2}}

I = π 2 384 \implies \displaystyle I = \dfrac{\pi^{2}}{384}

Ramiel To-ong
Jan 20, 2016

nice problem.

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